It is now common to use automated machine tool equipment to make parts for use in the automotive and other industries. Typically, the parts are secured to pallets, which are heavy metal structures, and the pallets are moved from station to station so that the workpiece can receive the various machining operations. Most automated machine tool equipment now in use utilizes transfer bars which reciprocate in a horizontal direction for transferring pallets (and the mounted workpieces) from one station to the next.
Conventionally, there are two modes for transferring these pallets:
(a) a lift and carry mode, in which a plurality of pallets are picked up at the work stations, lifted a predetermined amount off the rails, advanced by the distance between stations, and set back down on the rails;
(b) a slide mode in which a plurality of pallets is trapped at both ends at the various work stations, and then advanced one station at the time by sliding them on the rails.
The advantage of (a) lies in the fact that by lifting and lowering the pallets, no rail wear occurs, thus maintaining the machining accuracy. The disadvantage of (a) lies in the fact that chips or swarf which are part of the machining process may get trapped under the pallets, resulting in inaccurate location at the work station, even to the extent of causing malfunction at the work stations.
The advantage of (b) lies in the fact that, by sliding the pallet, chips and swarf cannot enter between the pallet and the rails, but instead are pushed off the rails into so-called dirt grooves during transfer. The disadvantage of (b) obviously is due to this very sliding motion which causes rail wear from metal fatique, hence loss of accuracy at the work stations.